Debt Securities (Tables)

3 Months Ended

Mar. 31, 2025

Investments, Debt and Equity Securities [Abstract]

Investments in Debt Securities, Classified as Available-For-Sale

nvestments in debt securities, classified as available-for-sale, are as follows:

	Amortized		Gross unrealized					Estimated
(in millions)	cost		Gains		Losses			fair value
March 31, 2025
U.S. Treasury bonds	$	175.5	$	0.5	$	(3.1	)	$	172.9
Municipal bonds		864.2		3.9		(71.6	)		796.5
Foreign government bonds		215.8		2.1		(5.2	)		212.7
Governmental agency bonds		202.2		—		(9.7	)		192.5
Governmental agency mortgage-backed securities		4,848.2		10.4		(249.8	)		4,608.8
U.S. corporate debt securities		981.9		7.7		(23.1	)		966.5
Foreign corporate debt securities		486.8		6.6		(9.3	)		484.1
	$	7,774.6	$	31.2	$	(371.8	)	$	7,434.0
December 31, 2024
U.S. Treasury bonds	$	180.7	$	0.1	$	(5.2	)	$	175.6
Municipal bonds		844.9		5.3		(73.7	)		776.5
Foreign government bonds		217.1		1.6		(7.1	)		211.6
Governmental agency bonds		203.8		—		(14.0	)		189.8
Governmental agency mortgage-backed securities		4,844.4		1.7		(343.8	)		4,502.3
U.S. corporate debt securities		948.4		5.6		(28.4	)		925.6
Foreign corporate debt securities		491.6		5.3		(12.4	)		484.5
	$	7,730.9	$	19.6	$	(484.6	)	$	7,265.9

Gross Unrealized Losses on Investments in Debt Securities

Investments in debt securities in an unrealized loss position, and their respective length of time in such position, are as follows:

	Less than 12 months					12 months or longer					Total
(in millions)	Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses
March 31, 2025
U.S. Treasury bonds	$	38.4	$	(0.7	)	$	45.8	$	(2.4	)	$	84.2	$	(3.1	)
Municipal bonds		268.0		(5.8	)		412.0		(65.8	)		680.0		(71.6	)
Foreign government bonds		4.4		—			63.7		(5.2	)		68.1		(5.2	)
Governmental agency bonds		146.3		(1.8	)		37.8		(7.9	)		184.1		(9.7	)
Governmental agency mortgage-backed    securities		2,558.4		(52.1	)		1,283.8		(197.7	)		3,842.2		(249.8	)
U.S. corporate debt securities		415.7		(5.5	)		118.1		(17.6	)		533.8		(23.1	)
Foreign corporate debt securities		153.5		(1.7	)		97.8		(7.6	)		251.3		(9.3	)
	$	3,584.7	$	(67.6	)	$	2,059.0	$	(304.2	)	$	5,643.7	$	(371.8	)
December 31, 2024
U.S. Treasury bonds	$	81.8	$	(1.8	)	$	52.9	$	(3.4	)	$	134.7	$	(5.2	)
Municipal bonds		248.3		(4.4	)		417.0		(69.3	)		665.3		(73.7	)
Foreign government bonds		29.8		(0.2	)		72.7		(6.9	)		102.5		(7.1	)
Governmental agency bonds		144.4		(5.3	)		37.9		(8.7	)		182.3		(14.0	)
Governmental agency mortgage-backed    securities		2,977.2		(98.4	)		1,290.4		(245.4	)		4,267.6		(343.8	)
U.S. corporate debt securities		435.2		(9.6	)		117.5		(18.8	)		552.7		(28.4	)
Foreign corporate debt securities		159.4		(3.2	)		110.5		(9.2	)		269.9		(12.4	)
	$	4,076.1	$	(122.9	)	$	2,098.9	$	(361.7	)	$	6,175.0	$	(484.6	)

Investments in Debt Securities by Contractual Maturity

Investments in debt securities at March 31, 2025, by contractual maturities, are as follows:

(in millions)	Due in one year or less		Due after one through five years		Due after five through ten years		Due after ten years		Total
U.S. Treasury bonds
Amortized cost	$	38.7	$	106.6	$	4.7	$	25.5	$	175.5
Estimated fair value	$	38.6	$	105.3	$	4.8	$	24.2	$	172.9
Municipal bonds
Amortized cost		13.0		85.7		286.1		479.4		864.2
Estimated fair value		13.1		83.2		256.2		444.0		796.5
Foreign government bonds
Amortized cost		8.2		125.7		78.2		3.7		215.8
Estimated fair value		8.2		127.7		74.1		2.7		212.7
Governmental agency bonds
Amortized cost		8.6		1.4		148.8		43.4		202.2
Estimated fair value		8.6		1.4		147.0		35.5		192.5
U.S. corporate debt securities
Amortized cost		14.1		555.9		319.5		92.4		981.9
Estimated fair value		14.1		554.0		316.9		81.5		966.5
Foreign corporate debt securities
Amortized cost		24.1		308.5		122.9		31.3		486.8
Estimated fair value		24.1		309.6		122.0		28.4		484.1
Total debt securities (excluding mortgage-backed    securities)
Amortized cost	$	106.7	$	1,183.8	$	960.2	$	675.7	$	2,926.4
Estimated fair value	$	106.7	$	1,181.2	$	921.0	$	616.3	$	2,825.2
Total mortgage-backed securities
Amortized cost										4,848.2
Estimated fair value										4,608.8
Total debt securities
Amortized cost									$	7,774.6
Estimated fair value									$	7,434.0

Composition of Debt Securities Portfolio by Credit Rating Agencies

The composition of the debt securities portfolio at March 31, 2025, by credit rating, is as follows:

	A- or higher					BBB+ to BBB-					Non-Investment Grade					Total
(dollars in millions)	Estimated fair value		Percentage			Estimated fair value		Percentage			Estimated fair value		Percentage			Estimated fair value
U.S. Treasury bonds	$	172.9		100.0	%	$	—		—	%	$	—		—	%	$	172.9
Municipal bonds		775.9		97.5			20.3		2.5			0.3		—			796.5
Foreign government bonds		207.5		97.6			4.5		2.1			0.7		0.3			212.7
Governmental agency bonds		192.5		100.0			—		—			—		—			192.5
Governmental agency mortgage-    backed securities		4,608.8		100.0			—		—			—		—			4,608.8
U.S. corporate debt securities		476.4		49.3			326.0		33.7			164.1		17.0			966.5
Foreign corporate debt securities		254.2		52.5			186.0		38.4			43.9		9.1			484.1
	$	6,688.2		90.0	%	$	536.8		7.2	%	$	209.0		2.8	%	$	7,434.0

Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies

The composition of the debt securities portfolio in an unrealized loss position at March 31, 2025, by credit rating, is as follows:

	A- or higher					BBB+ to BBB-					Non-Investment Grade					Total
(dollars in millions)	Estimated fair value		Percentage			Estimated fair value		Percentage			Estimated fair value		Percentage			Estimated fair value
U.S. Treasury bonds	$	84.2		100.0	%	$	—		—	%	$	—		—	%	$	84.2
Municipal bonds		664.5		97.8			15.2		2.2			0.3		—			680.0
Foreign government bonds		65.9		96.8			1.5		2.2			0.7		1.0			68.1
Governmental agency bonds		184.1		100.0			—		—			—		—			184.1
Governmental agency mortgage-    backed securities		3,842.2		100.0			—		—			—		—			3,842.2
U.S. corporate debt securities		279.3		52.3			168.1		31.5			86.4		16.2			533.8
Foreign corporate debt securities		147.3		58.7			78.0		31.0			26.0		10.3			251.3
	$	5,267.5		93.3	%	$	262.8		4.7	%	$	113.4		2.0	%	$	5,643.7